Police have received complaints about Neil Lennon, believed to relate to allegations of abuse towards Rangers striker El Hadji Diouf. Celtic called the allegations 'defamatory and outrageous'. Photograph: Danny Lawson/PA

Strathclyde police has opened an investigation into Wednesday night's Old Firm derby after complaints made by the public against the Celtic manager, Neil Lennon, regarding alleged abuse of the Rangers striker El Hadji Diouf.

The Scottish Cup replay at Celtic Park proved notable after three red cards for Rangers players and a touchline bust-up between Lennon and McCoist, the visiting club's assistant manager.

The police have now received a series of complaints about Lennon, believed to relate to allegations of abuse towards Diouf in what was a separate touchline clash, and are looking into them.

A Strathclyde police spokesman said: "We have received complaints from members of the public regarding allegations surrounding events during the Old Firm match on Wednesday 2 March 2011, which we are currently investigating."

"This is a concerted campaign by malicious and despicable individuals to cause distress to Neil Lennon.

"Celtic Football Club and Celtic fans stand by their manager in the face of these vicious and unacceptable attacks."

A suspicious package addressed to Lennon was intercepted at a Royal Mail sorting office on Friday. Security around the Celtic manager has been increased amid a series of incidents, which include another parcel — which contained bullets — being sent to him and late night, threatening phone calls.